1. Field of the Invention
This invention relates to quasi-optic grid arrays, and, in particular, to techniques for controlling the distribution of dc bias across such arrays and improved arrays incorporating such techniques.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the transmission of radio frequency (xe2x80x9cRFxe2x80x9d) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called xe2x80x9cpower combiningxe2x80x9d has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, spatial power combining using xe2x80x9cquasi-opticsxe2x80x9d was proposed as a potential solution to these problems. The theory was that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, minimizing conductor losses. Furthermore, a planar array could be fabricated monolithically and at shorter wavelengths, thereby enabling potentially thousands of devices to be incorporated on a single wafer.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.
It is believed that the first quasi-optical grid array amplifier was a grid developed by M. Kim et al at the California Institute of Technology. This grid used 25 MESFET differential pairs, demonstrating a gain of 11 dB at 3 GHz. As shown in FIG. 1, a typical grid amplifier 10 is an array of closely-spaced differential pairs of transistors 14 on an active grid 12 sandwiched between an input and output polarizer, 18, 24. An input signal 16 passes through the horizontally polarized input polarizer 18 and creates an input beam incident from the left that excites rf currents on the horizontally polarized input antennas 20 of the grid 12. These currents drive the inputs of the transistor pair 14 in the differential mode. The output currents are redirected along the grid""s vertically polarized antennas 22, producing a vertically polarized output beam 30 via an output polarizer 24 to the right.
The cross-polarized input and output affords two important advantages. First, it provides good input-output isolation, reducing the potential for spurious feedback oscillations. Second, the amplifier""s input and output circuits can be independently tuned using metal-strip polarizers, which also confine the beam to the forward direction. Numerous grid amplifiers have since been developed and have proven thus far to have great promise for both military and commercial RF applications and particularly for high frequency, broadband systems that require significant output power levels (e.g.  greater than 5 watts) in a small, preferably monolithic, package. Moreover, a resonator can be used to provide feedback to couple the active devices to form a high power oscillator.
One challenge in the design of active grid arrays relates to DC biasing of the differential pair cells in the array. Unfortunately, since any practical design requires the whole array to be biased with a single bias bus, one specific problem that affects the operation of active grid arrays is the uneven distribution of bias voltage across a given array of cells or a row of cells in an array. Due to the finite conductivity and limited thickness of metal lines used in actual circuits, grid arrays that require appreciable bias current to be distributed among many active devices (e.g., transistors) from a single source at the edge of the grid may suffer from unequal distribution of DC power. Specifically, the cells in the middle of the array tend to get less bias current that those closer to the dc source. This DC power distribution problem limits the effectiveness of large, high-power grid array components (e.g., grid amplifiers and grid oscillators).
It would thus be desirable to have a grid array in which the power distribution across the entire array could be controlled, by either providing an equal distribution of DC bias to each cell in the array or by providing a different, but desired, power distribution at the control of the array designer. The present invention fulfills this need.
The present invention, which addresses this need, resides in a method of controlling the distribution of a bias in a periodic array of electronic devices, such as a quasi-optic grid array. The method includes the steps of first sensing the voltage drop on the bias line resulting from the intrinsic resistance in the bias line at either all or selected electronic devices in the array, and then applying a prescribed bias voltage via an impedance network at the control input of either selected or all devices in the array based upon the sensed voltage drop. In this way, the bias voltages to each control input of either the selected or all electronic devices in the array can by tailored and thus will not suffer from an undesirable, non-ideal, and/or uneven bias distribution across the array.
More particularly, a method of controlling the distribution of bias in a periodic array of active cells that is biased with a DC bias supply, a bias supply line and a bias-return conductor line, is disclosed. The method includes sensing the voltage difference between consecutive cells on the bias-return conductor line resulting from the internal conductor resistance in the bias-return conductor line; and based on the sensed voltage difference, applying to each cell a prescribed control bias voltage. The prescribed control voltage is applied via an impedance network at each cell along the array that allows the control bias voltage to track the voltage difference on the return line.
The impedance network of the present invention includes a voltage divider circuit disposed between the bias-return current line and a reference voltage conductor return line, and a tracking resistance inserted in the reference-voltage conductor return line and is connected to the voltage divider circuit.
In one preferred embodiment, the impedance network maintains a relatively constant control bias voltage distribution to each cell in the array relative to the sensed voltage on the bias return line at each cell. In this way, the input control biasing voltage (Vgs in the case of FET""s) is the same across the entire grid array, regardless of the size of the array. In one specific embodiment, the active cells are differential pair amplifiers and the periodic array is a grid amplifier.
An improved periodic active grid array is also disclosed by the present invention. The array includes (a) a plurality of active cells that are combined at their outputs in a periodic arrangement via bias-supply conductors; (b) a DC bias voltage source that supplies bias to each of the cells; (c) a bias return conductor line connected to each cell, the line having internal resistances, Rcond, disposed along the bias-return conductor line; (d) a reference voltage source that supplies a voltage to the bias return conductor line; and (e) a bias distribution impedance network disposed between the bias return conductor line and an auxiliary reference-voltage return conductor line that controls the distribution of bias in a periodic array. In the preferred embodiment, the bias distribution impedance network includes three resistors. A first resistor is disposed between the bias return conductor line and a control input of a cell, and a second resistor is connected to the first resistor and the control input and at the other end to the reference-voltage conductor line. The two resistors define a voltage divider circuit that establishes, at each cell, the voltage at the control lead relative to the varying voltages on the bias-return conductor line. The third resistor is a tracking resistor, Rtrack, disposed between the second resistor of each cell and the reference-voltage conductor line, that causes the voltage on the reference-voltage control line at each cell to have a specific relationship to the voltage on the bias-return line.
In the embodiments where the control input bias is desired to be at least substantially constant throughout the array, the value of each Rtrack is defined by the equation: C(Rtrack+Rcond)=(1xe2x88x92C)Rcond, where C is a predetermined fraction of the current that flows from the main current return line that may be diverted.
In yet an even more particular aspect of the invention, an improved quasi-optic grid array that amplifies an RF input beam in free space is disclosed. The array includes (a) a plurality of differential pair unit cells, each having two control inputs that receive the input beam, two outputs that radiate the amplified beam and common cathodes, the cells being interconnected at their outputs in a periodic arrangement via bias-supply conductor lines, and a bias-return conductor line; (b) a DC bias voltage source connected to the bias supply conductor lines that supplies output bias to each of the cells; (c) a reference voltage source. The bias return conductor lines have an internal resistance, Rcond, disposed along the bias-return conductor line. Finally, a bias distribution impedance network is introduced between the bias return conductor line and an auxiliary reference-voltage return conductor line that controls the distribution of bias in a periodic array.
Finally, in another aspect of the present invention, the periodic active grid array of the present invention includes means for establishing, at selected cells, the voltage at the control lead relative to the varying voltages on the bias-return conductor line, and means for controlling a prescribed control bias voltage to be applied to each cell.